Extraordinary co-operation by sociable weavers, which work together to build the largest nests in the world, is motivated by family ties, say scientists.

New research, published in Ecology Letters, says the birds, which are found throughout southern Africa, are more likely to maintain the communal part of the nest if they have relatives living nearby.

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Dr Rene van Dijk from the University of Sheffield, one of the study's authors, compares the scenario to having lodgers to stay in the family home.

'If the lodger isn't related to the family, he or she may pay rent, but they will not care too much about the upkeep of the house,' he says.

'However, if the lodger is a known family member, then you would expect them to maintain the house which he or she may stay in for a longer period and possibly inherit. It may seem like a small difference, but it tips the balance towards a more co-operative society.'